The use of optical fibers in fiber optic transmission systems has prompted the use of single fiber attenuators which allow high power transmission signals to be processed directly into signal application apparatus. In the past, such attenuators have reduced signal levels by transmitting such signals through partially opaque optical filters, by obstructing a section of the signal with attenuation filter segments, by varying the orientation between signal input and output fibers, or by diminishing signal strength via reflection from partially light absorbent elements.
For example, in U.S. Pat. No. 4,591,231, Kaiser et al. describe an optical attenuator containing a disk having a plurality of openings with fixed, neutral density filters of differing optical density contained therein. By rotating the disk, a fixed density filter of a desired opacity can be introduced into the signal path, thereby attenuating a portion of the signal. Although effective for reducing signal strength, such a system is limited in :hat attenuation levels vary in a step-wise rather than a continuous manner.
U.S. Pat. No. 4,702,549 to Duck et al. teaches a variable attenuator in which an attenuation filter is used to obstruct a segment of the transmission signal. Such attenuators tend to suffer from internal refraction and reflection problems, thereby resulting in high signal noise which reduces the effectiveness of the device.
Attenuators in which the orientation between signal input and output fibers is varied have been described in U.S. Pat. No. 4,645,294 to Oguey et al. and in U.S. Pat. No. 4,145,110 to Szentesi. In the former patent, a first optical fiber is mounted in a fixed position, while a second optical fiber is mounted on a movable surface in a manner which allows its terminal end to be moved in a circular arc about the terminal end of the fixed fiber. In the latter patent, the terminal ends of the signal input and output fibers are mounted in an apparatus which allows the axial distance between the terminal ends to be varied.
Reflective means to vary signal attenuation have been described in U.S. Pat. No. 4,664,484 to Hines and in U.S. Pat. No. 4,364,639 to Sinclair et al. This latter patent also describes the use of a liquid crystal cell to vary the transmissity of a medium through which the transmission signal passes.
Additionally, means for providing fixed attenuation are well known in the art. Such systems generally utilize a medium of fixed optical density disposed between a signal input fiber and a signal output fiber. These media can comprise isolated material placed between the signal carrying fibers as described in U.S. Pat. No. 4,257,671 to Barbaudy et al., or can comprise a coating upon the terminal end of at least one of the signal carrying fibers as described in U.S. Pat. No. 4,639,079 to Sheem.